There are many instances in which it is desirable to laminate outer films to each other, especially films that have outer protective layers of a tough abrasion resistant material and an inner layer of thermoplastic bonding resin.
One commonly used outer protective film is of composite construction having one surface formed of a tough abrasion resistant polyester resin, the opposite surface comprising a material that serves as an adhesive for bonding the film to another film or to another film and an interior core. The most common adhesive material used is polyethylene.
One prevalent problem in laminating a composite assembly having outer films of plastic such as the composite film just mentioned is transverse curling of the composite due to heating of the films which is required to activate the thermoplastic bonding resin. This problem is due to minor variations in the film thicknesses, differences in the heating of the two laminating films and other factors. The resulting curl in these laminated articles, which occurs in the transverse direction, is frequently quite objectionable.
Transverse curling also is a problem in the manufacture of composite articles of such films which also have a central core. The central core can be an article to be protected or it can be a resilient material that normally keeps the outer films apart, at least in selected areas, usually areas that are coated with an electrically conductive material so that the composite article when equipped with suitable wiring can function as an electrical switch.